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91

Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by pokey on January 11, 2025, 09:51:02 pm »

If you ever suspect a centre gasket or RH main seal you will get a smokey exhaust as gearbox oil will also seep into the crankcase ,The gearbox oil will be contaminated and look a bit milky and stink a bit.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by 2T on January 11, 2025, 09:07:56 pm »

WR500 (YZ490 leftovers sold as WR500) the old YZ490 suffered from an inefficient head combustion squish i believe, they were notorious detonators.
i think its well documented what mods needs to be done on the squish to achieve good combustion, i don't know the answer but i read it a few times over the years and forget....too many beers ago 

Yep, am currently using a variation of the common Eric Gorr style head mod.

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Tech Talk / Re: PVL Timing

« Last post by Hoony on January 11, 2025, 07:20:58 pm »

I love this Forum. 

Me too, I don't know why more people don't contribute with either questions or answers.

probably because its virtually inactive these days, i used to check it out daily sometimes twice a day, now its every week or 2. as its slow with activity this is the reason not many check it out i reckon.

long gone are the days of the infamouse suzuki footpeg never ending story and the long threads on bikes like the sprite / american eagle / alron saga etc.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by Hoony on January 11, 2025, 07:16:09 pm »

WR500 (YZ490 leftovers sold as WR500) the old YZ490 suffered from an inefficient head combustion squish i believe, they were notorious detonators.
i think its well documented what mods needs to be done on the squish to achieve good combustion, i don't know the answer but i read it a few times over the years and forget....too many beers ago 

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by 2T on January 11, 2025, 01:33:26 pm »

Some good info there pokey, thanks.

Bike is a Yamaha WR500 (owner since 2004), set up for road use (supermoto/back road blasting) so have been asking it to do things it's not designed for.

Bike was (professionally) fully rebuilt 12 mths ago, including lapping all crankcase/cylinder/head join surfaces. Have had ongoing issues with detonation, tuning and high operating temps. This gearbox issue is just one more hard-to-explain event. Pretty sure the bike is cursed.


Re: bottom end charge sucked not pushed. Didn't know that. Thanks!

Re: g'box overfilled or breather blocked. Neither of those.

Re: hydrolocking. Do have a fair bit of oil around the crank but pretty sure it's not filling with fuel mixture and is not ultra smoky after start-up.

For the record, the bike is definitely not neglected in terms of maintenance and upkeep, quite the opposite. Having said that, I'm not a mechanic, but (probably because I've had so many dramas with this bike) I do now try and learn as much as I can about how these old 2 strokes work.

Also, have just had crankcases checked for microcracks and both are fine. Will get it back together and try again.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by pokey on January 10, 2025, 08:41:33 pm »

Pressure isnt actually an issue in the bottom end of a 2T engine as the charge isnt pushed its sucked, either by the pressure differential created in the combustion chamber or pipe scavenging. Maybe a whole 7PSI in the bottom end at most so if it holds a 10PSI (never any more or you will blow a gasket or seal) in a test for 10 mins its good to go. As for the gearbox leak my thoughts would be over full or a blocked breather.

Sounds like that engine needs a lot of love whatever it is so back to basics. Pull the clutch cover and clean the old gasket and crap off it and pull the locating pins then hone it flat with a sheet of fine wet and dry on a sheet of window glass so you know its flat. Then a new gasket or yamalube as per the specific factory requirements.

Sorry dont mean to be rude and basic but I gotta ask so please forgive me and remember we dont know what you have as you havnt said (WHAT bike do you have) and we dont know your experience or the bike history so again please forgive if i have over simplified and stated the obvious. The only way a bottom end can build pressure to blow a gearbox seal is if it is or close to hydrolocking. Bad float needles and bad fuel taps drip fuel while the bike is sitting and "SH#T" all runs down hill and you end up with an oily premix filling your bottom end halving the area. It will still run sort of after coaxing but be a Biatch to tune and smoke more on start up than a 13 year old with a hard on with his skills trying to impress a new girlfriend untill it starts to clear a bit and still run like shyte.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by 2T on January 10, 2025, 07:33:09 pm »

Thanks guys!

Are there any known immediate detrimental effects possible, like extra pressure in the crankcase?

I'm asking these questions as I'm trying to work out why I had gearbox oil forced out the clutch cover gasket and centre crankcase join immediately after I had been testing with overly rich main jet and spluttering (breather tube is not blocked).

Since then, a leak down test has shown no leaks, so it's a bit of a mystery at this stage. Next on the list is checking for cracks in the crankcase that may be opening up after reaching peak operating temperature.

Thanks again.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by pokey on January 10, 2025, 06:10:34 pm »

If, when attempting to safely fine tune 2 stroke carb jetting (specifically the main jet), using the method of jetting much richer to the point of spluttering, before then gradually changing to leaner jets until suitable jetting is achieved, what activity is happening in the motor to cause the spluttering (assuming all else is functioning correctly)?

Spluttering is an incomplete burn of the charge and this can occur at any point in the RPM range for various reasons as i had mentioned not just too rich on the main.
If as you state everything else is fine and your sure its a main jet issue the amount of fuel/Air ratio is too fuel rich which has an excessive cooling effect and the charge cannot complete its burn within its duty cycle. Its very similar to a flooded engine or running with the choke on. 3 things in a balance for a fire to burn, Fuel Heat and Oxygen, decrees one(Air) or two items(cooling) and no more fire its a simple as that .. As the boys have stated this results in oily carbons being produced which clog pipes gum ring landings foul plugs and crap performance. I hope this helps.

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by skypig on January 10, 2025, 11:01:26 am »

My guess:

It’s too rich to ignite/burn reliably. Possibly momentarily fouling the plug (bridging the gap with liquid)?
So the spluttering is a stroke where it didn’t light off?
Or potentially the mixture is burning more like a liquid than a gas?

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Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« Last post by John Orchard on January 10, 2025, 08:40:54 am »

The issues that could arise ....

Fouled plug.
Blocked baffles in the exhaust.
Excessive carbon build-up behind rings.
Ring might get jammed in the grooves.